This invention relates to method and means for determining the vertical plane incluing the axis of spin of spinning ballistic projectiles, in appreciation of the problem that in conditions of zero gravity conventional means for determining the vertical, such as pendulum devices, are not effective.
During ballistic flight, a projectile tilts about a horizontal pitch axis which is perpendicular to its direction of flight. The projectile executes the first part of its trajectory tilted in a nose-up attitude or attitudes, and subsequently, in the latter part of the trajectory in a nose-down attitude or attitudes. The projectile thus rotates about the horizontal pitch axis. In order to cause the projectile to depart fom its ballistic trajectory, for example to execute a `pull up` maneuver, the projectile must be tilted about this axis to alter its angle of incidence, that is to asy its pitch axis is tilted with respect to its trajectory or line of flight.
Naturally, this change of angle of incidence or tilting movement must be effected whether the projectile is spinning or is de-spun so that the projectile is properly controlled.
It is therefore an object of the invention to provide method and means to indicate the vertical plane through the projectile axis and also the projectile's position with respect to that plane at any stage in flight.
According to one aspect of the invention, there is provided a method of determining the position of the vertical plane extending through the spin axis of a spinning ballistic projectile in flight, the method comprising the steps of:
obtaining signals corresponding to the acceleration experienced along a sensitive axis parallel to but spaced from the spin axis, said signals containing periodic and non-periodic terms, the periodic term being related to the spin frequency,
processing said signals to isolate the periodic term, and
ascertaining the phase of said signal with reference to a datum so that an indication of the vertical plane is obtained.
According to a further aspect of the invention, a projectile which spins in ballistic flight and which requires the position of the vertical plane extending through the spin axis to be determined includes
accelerometer means mounted with its sensitive axis parallel to but spaced from the spin axis to provide output signals indicative of the acceleration experienced along said sensitive axis, said signals containing periodic and non-periodic terms, the periodic term being related to the spin frequency, and
processing means for isolating the periodic term and ascertaining the phase thereof so that an indication of the vertical plane is obtained.